1. Field of the Invention
The present invention relates to a line switching system. More particularly, it relates to a line switching system that switches over a transmission path where a transmission line between nodes is disconnected or fault is found on an optical transmission device at nodes, in an optical transmission system in which a plurality of optical transmission devices are connected in a ring form via optical transmission lines.
2. Description of the Related Art
In recent years, large transmission capacity and flexibility of services have been desired, which can make the form of line connections flexible in an add/drop system of an optical transmission system.
At the same time, it becomes indispensable to use a line switching system which can deal with the fault of the transmission lines with a high speed, to improve reliability of line services.
FIG. 6 is a diagram for explaining a conventional line switching system in an optical transmission system, in which a plurality of nodes (#A to #D) are connected in a ring form via optical transmission lines. In FIG. 6, reference numerals 70 to 73 are nodes (#A to #D) in the optical transmitting system.
The nodes (#A to #D) 70 to 73 are connected in a ring form by optical transmission lines 75 to 78. Further, optical transmission lines 75 to 78 transmit optical signals in both directions.
Each of the nodes is an optical transmission device, which multiples digital signals DSn to create paths #1 to #3, and transmits data of optical signals in EAST sections 701, 711, 721 and 731 and WEST sections 702, 712, 722 and 732, with an appropriate time slot carrier.
Each of the EAST sections 701, 711, 721, and 731, and the WEST sections 702, 712, 722 and 732 is a circuit having functions of OE/EO conversion, multiplication and fault detection.
In FIG. 6, the transmission lines 75 to 78 for the optical signals are shown with thick lines, and the transmission lines for the digital signals in each optical transmission device are shown with fine and dotted lines. The optical signal has time slots, at which paths #1 to #3 can be transmitted.
In the example of FIG. 6, the path #3 of the node 70, the path #1 of the node 71 and the path #2 of the node 71 are connected to the path #3 of the node 72, the path #1 of the node 73 and the path #2 of the node 73 for transmitting data, respectively.
In a normal state, data are transmitted in the direction shown with fine arrows. Thus, all of time slot carriers of the optical signal on the optical transmission line 78 between the node 70 and the node 73 are used for paths #1 to #3. The time slot carriers of the optical signal on the optical transmission line 76 between the node 71 and the node 72 are not used (shown with the broken lines in FIG. 6).
With such as a structure, the case where the optical transmission line 77 is disconnected at the fault point Px will be considered as follows. The same data is generally sent in both directions of the EAST section and the WEST section from the sending side, and data sent in either one of both directions is selected on the receiving side.
In the case where the optical transmission path is disconnected at the fault point Px, the system detects the disconnection in the WEST section 722 of node 72 and the EAST section 731 of node 73, and sends a switching signal AIS (Alarm Indication Signal) to all paths #1 to #3.
The paths #1 to #3, which received the switching signal AIS switch the receipt side to the reversed side. For example, path #3 switches from the direction of the side of EAST section 701 to that of WEST section 702 in node 70 shown in FIG. 6.
In the structure of the conventional line switching system as explained above, the switching is executed between the paths, so that it is required to reserve a protection line for switching, even between the nodes, which do not terminate the paths, namely, between nodes 71 and 72 in FIG. 6. That is the drawback of the conventional system, which would make transmitting efficiency worse.
To improve the conventional system of FIG. 6, a two-fiber switching system has been proposed, which has flexibility to select a switching route. In the system, APS (Automatic Protection Switch) byte, used as a redundancy data in a signal frame, is exchanged between the adjacent nodes hereinafter referred to as APS nodes, and switching is executed to a selected route according to the APS bytes, designated as K1 and K2 bytes as defined by the Bellcore Recommendation TR-NWT-00253.
FIG. 7 shows a basic structure of the 2-fiber switching system for switching with the APS bytes. Same reference numerals and symbols are used to denote and identify the elements corresponding to same elements shown in FIG. 6. In a normal state, a signal is transmitted in one direction on one of a pair of optical transmission lines 75 to 78.
In the case where the optical transmission lines are disconnected at the fault point Px shown in FIG. 7, node 72 and node 73 exchange data by using the APS bytes on the route which is not disconnected, the data from the node 73 to the node 72 is looped back to the route which is not disconnected (the side of WEST section of node 73), and the data from node 72 to node 73 is looped back to the route which is not disconnected (the side of EAST section of node 72), and the flows of the signals are turned in the reversed direction on the other pair of optical transmission lines 75 to 78.
In this system, it is also possible to detour signals by using the time slot carrier of the optical signal of another system as shown in FIG. 8. More particularly, in FIG. 8, reference numerals 80 and 81 show the nodes in the other system and which are connected by the optical transmission lines 82.
When the disconnection is found at the fault point Px between node 72 and node 73, the APS bytes are used between the appropriate APS nodes. The optical transmission lines 83 and 84 are connected to the path #1 of node 72 and the path #3 of node 73, respectively, so as to bypass between the fault point Px and another optical signal route reserved in advance, and idle time slots between node 80 and node 81 are used to detour the signals.
Further, with this system, there is no need to be conscious of the state for setting the lines of the entire system. Only finding a detour route is required for only the carrier sent and received between the APS nodes.
Thus it becomes possible to reduce the time required for switching and the amount of managing data.
However, such system has been realized with the line switching using the above-described APS bytes in the system in which a plurality of transmission devices are connected in a ring form.